Step 1: Sear the Chicken & Mushrooms (Highly Recommended)
Pat the chicken completely dry and season generously with salt and pepper. If using, dredge lightly in flour, shaking off excess. In a large skillet, heat 1 tablespoon of oil or butter over medium-high heat. Working in batches, sear the chicken for 3-4 minutes per side until golden brown. Transfer to the slow cooker. In the same skillet, add the remaining oil/butter. Add the sliced mushrooms and cook for 5-7 minutes until they release their liquid and brown deeply. Add the diced onion and cook for 3-4 minutes until softened. Add the garlic and cook for 1 minute until fragrant.

Step 2: Deglaze with Marsala
Pour the Marsala wine into the hot skillet with the mushrooms and onions. Scrape the bottom vigorously with a wooden spoon to release all the browned bits (fond). Let it simmer for 2-3 minutes to reduce slightly.

Step 3: Build the Braising Liquid
To the skillet, add the chicken broth, tomato paste, thyme, bay leaves, and soy sauce. Stir until the tomato paste is dissolved. Bring to a gentle simmer.

Step 4: Assemble in the Slow Cooker
Pour the entire contents of the skillet (mushrooms, onions, and all liquid) over the seared chicken in the slow cooker. Ensure the chicken is mostly submerged.

Step 5: The Long, Slow Braise
Cover and cook on LOW for 5-6 hours or on HIGH for 3-4 hours. The chicken is done when it is fork-tender. If using chicken breasts, check 1 hour earlier.

Step 6: Thicken the Sauce
Carefully remove the chicken to a plate and tent with foil. If the sauce is too thin, you can thicken it. Option A: Create a slurry with 2 tablespoons of cornstarch and 3 tablespoons of cold water. Whisk into the sauce in the slow cooker, cover, and cook on HIGH for 15-20 minutes until thickened. Option B: For a richer finish, turn the slow cooker to HIGH and let the sauce reduce uncovered for 20-30 minutes.

Step 7: Finish the Sauce
Once thickened to your liking, turn the slow cooker to WARM or off. Remove the bay leaves. Whisk in the cold butter, one cube at a time, until melted and incorporated—this creates a glossy, rich finish (a technique called monter au beurre). For a creamier version, stir in the optional heavy cream. Taste and adjust seasoning with salt and pepper.

Step 8: Serve and Garnish
Return the chicken to the sauce to warm through for a few minutes. Serve the chicken and mushrooms over your chosen base, spooning plenty of the luxurious Marsala sauce over the top. Garnish generously with fresh chopped parsley